Also, in 2010 their import is 44.9 billion dollars twice the amount they export, 21.1 billion. Even if their exports go up because of weaker currency, the pain from higher import prices will be much worse than the rises of income in the short run.
Your consequence number 1: trade barrier is an interesting possibility, but 2: the collapse of EU is unlikely to happen given that the Greek economy is only 3% of EU, and this strong measure will have threatened other troubled economies enough they will be much less likely to default and cause more issues. The measure might in fact prevent the collapse of EU at least in the short- to medium- term.
More jobs has a strong political effect too; people will probably tolerate more austerity if it's not combined with unemployment.
The article explains pretty well in my opinion why that 3% is very capable of collapsing the rest of the EU. If Greece defaults, investors lose their money. That's not too bad (although there could be a domino effect with banks), however investors will also start worrying more about a default of Ireland and Portugal, followed by Spain and Italy. Such worry translates to higher interest rates leading to the horror scenario of Spain needing a bailout (which noone can afford to give them).
Anything that amounts to Greece not repaying their debts, will be considered a default by the market, likely leading to what I described in the above paragraph.
Of course, the most mobile part of the population and most companies would emigrate under such circumstances.
Default (in some form) is a very attractive alternative, but the EU will keep coming up with better and better deals to prevent it.
Default is the only realistic option, but Germany and France refuse it for political reasons. The other reason is that a lot of Greek debt is owned by banks from those countries: selling banks bail-out in France or Germany is not that much more popular than in the US nowadays.
What is true is that Germany has put in place a devaluation strategy (by reducing wages instead of devaluating its money), but this has been a catastrophy for Europe as a whole. "Real" gdp growth comes from increase of productivity: devaluation helps hiding this for some time, but not that long.